This invention relates to the production of graphite fiber reinforced metals. A method is disclosed for the treatment of graphite fibers by molten sodium/potassium alloys (NaK) so that the fibers may be wetted and infiltrated by molten metals such as aluminum, magnesium, copper, tin, lead, zinc, and the like, as well as alloys thereof, thereby permitting the fabrication of improved graphite fiber reinforced metals at lower temperatures and shorter times than are currently required. The invention may be employed in the metal impregnation of woven graphite fiber preforms such as rings, cones, frusta and the like.
Graphite fibers exhibit extremely high tensile strengths and stiffness. Several types of graphite fiber are currently commercially available, being essentially partially graphitized carbon fibers, the degree of graphitization depending on the manufacturing method and the precursor material selected. Common precursors include polyacrylonitrile, pitch and rayon. The microcrystalline structures of the fibers show the basal planes of the graphite lattice lying parallel to the fiber axis. In rayon based fibers and polyacrylonitrile based fibers, the basal planes are circumferentially oriented, while in pitch precursor fibers the basal plane orientation is largely random with respect to the cross-section, but still parallel to the fiber axis. Composite materials containing graphite fibers in a matrix of another material such as a resin or a metal combine the physical properties of the graphite fiber, high tensile strength, stiffness and elevated temperature resistance, with the properties of the matrix material. Graphite metal composites not only exhibit the combined structural properties of graphite and the metal, but in addition are extremely light weight in relation to their strength, due to the relatively low density of graphite.
Graphite fiber reinforced metals are fabricated by immersing the fiber in a bath of the molten metal. A principal difficulty in the manufacture of graphite fiber reinforced metals arises from the fact that such metals as aluminum, magnesium, copper, tin, lead, zinc and various alloys thereof do not readily wet graphite fibers, making it difficult to impregnate the fibers with the molten metal. To overcome this problem, various pretreatments have been employed in the prior art. One pretreatment commonly used in the prior art involves deposition from the vapor phase of titanium and boron in combination. Another technique, more pertinent to the present invention, is disclosed in U.S. Pat. No. 3,770,488 to Pepper, et al., wherein the graphite fiber is first infiltrated with molten sodium before immersion in the molten bath of matrix metal. The graphite fiber must be immersed in a molten bath of liquid sodium maintained at 550.degree. C. under an atmosphere of argon for about 15 minutes. In a second step of the process, the fibers are removed from the molten sodium and transferred to, and immersed in, a molten bath of the desired metal matrix. After an appropriate immersion time, the composite fibers are removed from the alloy bath, and cooled to form graphite metal composite fibers. Regardless of which prior art method is used for pretreating the graphite fibers to make them wettable, the fabrication of more complex articles requires a third step, in which graphite metal composite fibers are woven into the final desired shape, which may then be clad with alloy, as by hot pressing.